Horticulture Principles and Practices

(This item omitted from WebBook edition)
FIGURE 1 Black plastic mulch
applied in the field on raised
beds for the production of
muskmelons. (Photo courtesy of
Dr Mike Orzolek – PSU)
crop was finished being harvested (multiple crops in southern states such as Florida
and Texas). As more acreage of plastic mulch film increased within the last 20 years,
it was apparent that new types of plastic mulch film would have to be developed to
help the growing problem of sending non-degradable plastic mulch to landfills. The
plastic film manufacturers answer to the problem was to produce a photodegradable
plastic mulch in the mid eighties which would degrade when the film was exposed to
light (specifically ultra violet light). After several years of trial in the field, two
problems became obvious; 1) the plastic film under the soil of the shoulders on the
bed did not degrade during the growing season since it was not exposed to the
UV light and 2) it was very difficult if not impossible to accurately determine/predict
when the photodegradable plastic film would degrade and become ineffective.
Photodegradable plastic film that should have lasted for 90 days in some cases lasted
only 30 days and film that should have started to breakdown in 40 to 50 days didn’t
degrade until 120 days. The unpredictability of the photodegradable plastic mulch
resulted in most plastic manufacturers dropping the product by the 1990’s. Within the
last 5 years in both Europe and North America, vegetable crop growers were looking
for a truly degradable plastic film that did not have to be retrieved from the field at
the end of the growing season. Plastic mulch manufacturers, especially in Europe,
developed a biodegradable plastic mulch that would be effective for 30 to 45 days
after being placed in the field before the mulch became brittle and started to breakdown
in the field. At the end of the growing season, the biodegradable plastic mulch can be
rototilled into the soil and a cover crop planted into that field 7 to 10 days later. The
biodegradable plastic mulch degrades in response to both moisture and microbial
activity in the soil.
The color of plastic mulch was initially either black or clear when introduced in
the sixties. Black was the preferred color because of the increase in soil temperature
compared to no plastic mulch and black eliminated photosynthetic active radiation
(PAR) from reaching the soil, thus eliminating weed growth. Clear plastic mulch
increased soil temperature at least 5°F to 8°F warmer than black mulch, but clear mulch
allowed for weed growth under the film. Within the last 10 years, other colors have been
introduced into the industry including red, Infrared Thermal green (IRT), IRT brown,
blue, metalized or reflective silver, white, and co-extruded films with two colors, black
on one side and white on the other side for example. The change in vegetable crop yield
in response to the different mulch colors is due in part to an increase in soil temperature
and the range of reflected wavelengths produced by individual mulch colors. Different
colors will absorb and reflect different wavelengths of light and plants are very sensitive
to the color of light their leaves intercept from the sun and reflected surfaces. Red
and far-red light (between 600 and 800 nanometers) produces the largest growth
responses in plants. Light that has a lower far-red to red (FR/R) ratio will cause a plant
to develop shorter stems and larger roots (Table 1). A higher FR/R ratio will cause a
plant to direct more new growth into shoots, resulting in a taller plant with more leaves.
Different mulch colors reflect different wavelengths of light and thus different FR/R
ratios (Figure 2).
20.3 Regionalized Production of Vegetables 615